This application is a U.S. national phase application based upon International Application No. PCT/SE00/01172, filed Jun. 7, 2000; which claims priority from Swedish Application No. 9902308-7, filed Jun. 18, 1999.
The present invention relates to a sealing arrangement for a pulp dewatering arrangement.
In production of paper pulp from cellulose-containing fibre material, it is necessary to wash and dewater the paper pulp at several stages in the process. A previously known and commonly used arrangement for washing and dewatering of paper pulp, called a wash press, is described in SE-C-380,300, SE-C-501,710, U.S. Pat. No. 5,488,900 and SE-C-504,011. The arrangements disclosed in these documents comprise two cylindrical rotatable screen members arranged in an essentially convergent trough. Other examples of known arrangements are disclosed in U.S. Pat. No. 4,543,161 and in U.S. Pat. No. 5,667,642, the last-mentioned representing an arrangement in which the screen members rotate in the opposite direction to the usual one, i.e. the right screen member rotates counter-clockwise and the left screen member rotates clockwise, as viewed from the side.
A problem encountered in washing and dewatering with wash presses of the abovementioned type is that fibre residues can remain on the drum after the washed and dewatered fiber mat has been removed. To flush these fiber residues away and to clean the holes in the screen members, spray pipes are used, for example those shown in U.S. Pat. No. 4,861,433, where the spray pipes flush the exposed surface of the cylindrical rotatable screen members after the washed and dewatered fibre mat has been scraped off by a doctor blade. In wash presses with 180 degrees coverage of the pulp web, the flushing takes place on the downwardly moving part of the drum, which means that the spray water and flushed-off fibre residues can run off. This has functioned satisfactorily, and repeated shutdowns for cleaning are not required. These spray pipes have shortcomings in their flushing capacity in high-power wash presses with longer dewatering paths of 270 degrees coverage, where pulp for dewatering is applied at the highest point of the drum and dewatered pulp is removed at the press nip between two drums. In such constructions, the spray water and the flushed-off fibre residues cannot run off along the drum naturally, since this flow would run back down towards the dewatered pulp. This means that fibre residues are continuously accumulated on the surface of the screen members against the pulp headbox until the wash press is buried.
This accumulation of fibre residues means that the wash press has to be cleaned at regular intervals. Examples of sealing arrangements on the pulp headbox are shown in U.S. Pat. No. 3,980,518 and SE-B-504,011. SE-B-503,010 shows a variant with resilient steel bands whose purpose is to allow pressurization of the pulp area and compatibility with wire cloths. Adjustment of the seal is said to be simple, as is its replacement when so required. The wear and tear on this type of seal is extensive and it needs to be replaced at more or less regular intervals.
One object of the present invention is to avoid the problems with known sealing arrangements in the headboxes of wash presses and to make available a wash press with improved sealing of the pulp headbox, which sealing can be easily adjusted depending on the pulp concentration in question and the pressure of the pulp in the pulp headbox.
Another object is to make available a wash press with improved sealing of the pulp headbox, which sealing allows fibre residues remaining on the surface of the circular screen members to pass the seal and onwards into the dewatering zone. This prevents fibre residues from accumulating against the sealing strip and instead being drawn into the dewatering or press area again. No pile of accumulating fibre residues builds up against the seal, which fibre residues otherwise lead to increased wear of the screen plate. The wash press can therefore be operated for longer periods of time without unnecessary stops for cleaning, and it is possible to dispense with complicated cleaning arrangements.
Yet another object is to make available a wash press with improved sealing of the pulp headbox, which seal itself is exposed to minimum continuous wear, and the wear on cooperating screen members is reduced to a minimum.
Yet another object is that the primary seal is achieved by the fact that a dynamically sealing plug with progressively increasing concentration of pulp is formed in the sealing gap. The dynamically sealing plug is also able to adapt continuously to changes in the sealing gap caused by the gap-forming plate changing position or the screen member springing aside. The dynamic effect is achieved by the fact that outflowing pulp along the lip is continuously thickened, turns and follows the screen plate back into the press again. The dynamic effect ensures that the loads and the wear which would otherwise be developed by a stationary fibre plug against the screen plate can be greatly reduced.